The present invention relates to semiconductor devices, and more particularly to backward diodes that may be used for microwave detection and mixing.
The tunnel diode is a well-known semiconductor device that conventionally includes two regions of heavily doped semiconductor material of opposite conductivity types separated by a relatively thin junction which permits charge carriers to tunnel through upon the application of a suitable operating potential to the semiconductor regions. The p and n regions of tunnel diodes are so heavily doped that they are degenerate. At equilibrium, a portion of the valence band in the p region of the diode is empty and part of the conduction band in the n region is filled.
Adding a slight back bias brings some filled energy levels of the valence band of the p region to empty energy levels of the conduction band of the n region and, consequently, electrons will flow from the p region to the n region by the quantum-mechanical tunnel effect. Because the number of available valence band energy levels increases with back bias, and since the distance over which the electrons must tunnel decreases with increasing back bias, the back current increases very rapidly with increasing back bias. This is known as the Zener effect. Since the direction of current flow is when the electrons flow from the n region to the p region, the above current is negative.
A slight forward bias brings some levels of the filled part of the conduction band of the n region to empty levels of the valence band of the p region. In this situation, quantum-mechanical tunneling allows electrons to flow from the n region to the p region, giving a positive current that first increases with increasing back bias. When the filled part of the conduction band of the n region is maximally aligned with the empty part of the valence band of the p region, the current goes through a maximum. Subsequently, the current decreases with increasing forward bias, and should approach zero if the filled part of the conduction band of the n region lies opposite the energy gap of the p region. When a yet larger forward bias occurs, electrons and holes are injected over the barrier into the p and n regions, respectively, resulting in a rapid increase in current for increasing forward bias. Thus, the current-voltage has a negative conductance part in the forward region of the characteristic.
In practice, the majority of tunnel diodes are manufactured using one of the following techniques: (1) ball alloy, in which a small metal alloy pellet containing a counter-dopant of high solid solubility is alloyed to the surface of a mounted semiconductor substrate (with high doping) in a carefully controlled temperature-time cycle under inert or hydrogen gas, with the desired peak current level obtained by an etching process; (2) pulse bond, in which the contact and the junction are made simultaneously when the junction is pulse-formed between the semiconductor substrate and the metal alloy containing the counter-dopant; or (3) planar processes, in which fabrication consists generally of the use of planar technology including solution growth, diffusion, and controlled alloying.
Tunnel diodes originated with a device commonly known as an xe2x80x9cEsaki diodexe2x80x9d, which was described by Leo Esaki in xe2x80x9cNew Phenomenon in Narrow Germanium p-n Junctionsxe2x80x9d, Phys. Rev., 199, 603, published in 1958. There he described, while studying the internal field emission in degenerate germanium p-n junctions, an anomalous current-voltage characteristic observed in the forward direction, which amounted to a negative resistance region over part of the forward characteristic. This characteristic was described through the use of a quantum tunneling concept and led the way to many devices based on its concept. The tunneling time is very short, which permits the use of tunnel diodes well into the millimeter-wave region.
If the doping levels of the p and n regions are decreased, the filled portion of the conduction band of the n region and the empty part of the valence band of the p region become narrower. Consequentially, there will be fewer energy levels from which tunneling under a forward bias can occur. Hence, the maximum in the forward current-voltage characteristic becomes lower and lower until it practically disappears at a given doping level. At this point, the rectifying characteristic becomes the opposite of that of a normal diode. Such a diode is therefore is known as a backward diode.
Backward diodes are utilized in applications such as microwave detection and mixing. Their advantages include high speed, linear square law behavior, zero or small required bias, and temperature insensitivity. The standard backward diode is made from a heavily doped Ge p-n homojunction as utilized in Esaki-type diodes. Ge diodes of this type are manufactured utilizing crude alloying and diffusion techniques that are difficult to control. Furthermore, the materials are not sufficiently stable to withstand normal temperatures when attaching to die, and epoxying is required. Therefore, it is an object of the present invention to provide a backward diode for which manufacture is easily controllable, which may be attached normally to a die, and which may be used as a more consistently manufacturable replacement for current Ge back diodes.
References that provide further background regarding backward diodes include,
C. A. Burrus, IEEE Transactions on Microwave Theory and Techniques, p. 357, September 1963, describing Ge back diodes and their characteristics as detectors;
T. A. Richard, E. I. Chen, A. R. Sugg, G. E. Hofler, and N. Holonyak, Applied Physics Letters 63, p. 3613 (1993), describing GaAs backward diodes with a 100 xc3x85 In0.1Ga0.9As layer; and
W. L. Chen, G. O. Munns, X. Wang, and G. I. Haddad, Proceedings IEEE Cornell Conference, August 1995, p. 465 (1995), describing InGaAs (on InP) uniformly doped (n=p=5xc3x971019 cmxe2x88x923) for conventional Esaki tunnel diodes.
The present invention provides a method for epitaxially-growing a backward diode device including the steps of epitaxially growing an n-side including at least one n-doped layer; epitaxially growing a thin, heavily n-doped layer on the n-side; and epitaxially growing a p-side opposite the n-side on the thin, heavily n-doped layer, said p-side including at least one p-doped layer. Furthermore, the method is preferably used when the at least one n-doped layer, the thin, heavily n-doped layer, and the at least one p-doped layer are grown of a semiconductor material, and optimally when the semiconductor material is InGaAs.